The present invention relates to amplifier circuits. More particularly, the present invention relates to a low power current feedback amplifier with a lower output impedance input stage buffer.
The increasing demand for higher performance amplifier circuits has resulted in the continued improvement of the precision and accuracy of the various devices and components within the amplifier circuits, as well the increased response and bandwidth, and lower power requirements, of the amplifier circuits. For example, current feedback amplifiers, such as those incorporated within various video and audio drivers, equalizing filters, ADC input drivers, and other communication devices, are being required to provide lower power with increased bandwidth.
It would be ideal if current feedback amplifiers had gain independent bandwidth; however, such is not the case in practical applications. One area that affects the bandwidth performance is the output impedance of the inverting input terminal of the current feedback amplifier. The inverting input terminal impedance of the current feedback amplifier typically ranges from moderate to high. This level of impedance increases as the quiescent current bias level within an input buffer of the current feedback amplifier is reduced.
For example, with reference to FIG. 1, a basic current feedback amplifier 100 having an input buffer 102, a pair of current mirrors 104 and 106, and an output stage 108 illustrated. Input buffer 102 provides input terminals INP and INN for current feedback amplifier 100. A pair of output terminals of input buffer 102 drive current mirrors 104 and 106, which can provide a mirrored current to output stage 108.
Input buffer 102 comprises an open loop buffer, such as that illustrated in FIG. 2, including bias currents I1 and I2, a pair of input diode-connected transistors Q1, and Q2, and a pair of output devices Q3 and Q4. As discussed above, ideally, the bandwidth of current feedback amplifier 100 is independent of the gain of current feedback amplifier 100; however, in reality, the bandwidth of current feedback amplifier 100 is dependent upon any compensation capacitance and the value of an external feedback resistor RFBext (not shown) that can be configured in a closed feedback loop with current feedback amplifier 100. The output impedance of input buffer 102 gets multiplied by the noise gain of current feedback amplifier 100 and adds to the external discrete feedback resistor, to give an equivalent feedback resistor. For example:
RFBext+(Noise Gain * Input Buffer Output Impedance)=RFBequiv
Equivalent feedback resistor RFBequiv determines the bandwidth of current feedback amplifier 100. Changing the noise gain of current feedback amplifier 100 in an effort to try and maintain bandwidth requires changes in the value of equivalent feedback resistor RFBequiv. As a result, the bandwidth is never constant due to other effects on the output impedance of input buffer 102. In addition, for low power applications, the higher output impedance becomes even more problematic, i.e., lower quiescent current results in higher output impedance.
One attempt to solve the higher output impedance problems of input buffer 102 includes the increasing of biasing currents I1 and I2 to reduce the output impedance of input buffer 102. However, increasing biasing currents I1 and I2 increases the quiescent power requirements, which is undesirable.
In accordance with various aspects of the present invention, an overall low power current feedback amplifier having a lower output impedance input stage is provided. To reduce the output impedance, the input stage comprises a closed-loop input buffer. The closed-loop input buffer realizes a low output impedance since the loop gain of the input buffer reduces the output impedance of the input buffer. With a lower output impedance, the bandwidth of the current feedback amplifier becomes more independent of the gain, even at low supply current implementations.
An exemplary input buffer can be configured in a current feedback amplifier or a voltage feedback amplifier arrangement. In accordance with an exemplary embodiment, the input buffer comprises another closed-loop current feedback amplifier configured within the overall current feedback amplifier, wherein the output of the input buffer corresponds to the inverting node of the overall current feedback amplifier. The closed-loop configuration of the input buffer is facilitated by the use of an internal feedback resistor coupled from an inverting input terminal of the input buffer to the output of the input buffer, which corresponds to the inverting input terminal of the overall current feedback amplifier. Thus, with a low impedance at the inverting input terminal, the overall current feedback amplifier realizes a gain independent bandwidth over a wide range of supply currents.